This invention relates generally to a sensor system for remote detection and imaging of objects in a backscattering medium such as water. More particularly, this invention relates to a method and apparatus for detecting, locating and/or imaging underwater objects in shallow water and in coastal regions from an airborne platform using a novel imaging lidar (light detection and ranging) system which improves imaging in such shallow water areas.
It is desirable in a number of military and civilian applications to search a volume within a backscattering medium for the presence of certain targets. For instance, moored or bottom mines deployed in ocean shipping lanes are a hazard to navigating ships used both for military and for commercial purposes. For other civilian applications such as law enforcement on the ocean, it is desirable to detect the presence of submerged fishing nets or drug-carrying containers used in smuggling contraband. In or near harbors and beaches, it is also desirable to detect submerged obstructions, cables, pipelines, barrels, oil drums, etc. In strictly military applications, anti-submarine warfare demands an effective means of detecting and locating submarines.
Presently, cumbersome and time consuming wire line devices must be used for detecting underwater targets from remote airborne locations. These devices are lowered into the water and are easily subject to damage and loss. Also, wire line devices make target searching relatively slow and can only detect targets without providing visual imaging.
An improved and novel system for remote detection and imaging of objects underwater (or objects obscured by other backscattering media which are at least partially transmitting to light such as ice, snow, fog, dust and smoke) from an airborne platform has been described in U.S. Pat. No. 4,862,257 and U.S. Pat. No. 5,013,917, both of which are assigned to the assignee hereof and incorporated herein by reference. The imaging lidar system of U.S. Pat. No. 4,862,257 utilizes a laser to generate short pulses of light with pulse widths on the order of nanoseconds. The laser light is expanded by optics and projected down toward the surface of the water and to an object or target. U.S. Pat. No. 5,013,917 relates to an imaging lidar system intended for night vision.
Imaging lidar systems of the type described hereinabove are also disclosed in commonly assigned U.S. Pat. No. 4,964,721 and U.S. Pat. No. 4,967,270, both of which are incorporated herein by reference. U.S. Pat. No. 4,964,721 relates to an imaging lidar system which controls camera gating based on input from the aircraft onboard altimeter and uses a computer to thereby adjust total time delay so as to automatically track changing platform altitude. U.S. Pat. No. 4,967,270 relates to a lidar system employing a plurality of gated cameras which are individually triggered after preselected time delays to obtain multiple subimages laterally across a target image. These multiple subimages are then put together in a mosaic in a computer to provide a complete image of a target plane preferably using only a single light pulse.
Still other imaging lidar systems are disclosed in commonly assigned U.S. Pat. Nos. 5,029,009 and 5,034,810, both of which are incorporated herein by reference. U.S. Pat. No. 5,029,009 describes an imaging lidar system incorporating an imaging camera having a plurality of gating electrodes on a focal plane and means for producing variable time delay gating across an image viewed by the focal plane. U.S. Pat. No. 5,034,810 relates to a two wavelength lidar imaging system for underwater application utilizing image subtraction to detect passage of internal waves or other anomalies under water.
U.S. Ser. No. 07/565,631, filed Aug. 10, 1990 (now U.S. Pat. No. 5,231,401) which is also assigned to the assignee hereof and fully incorporated herein by reference, relates to an airborne imaging lidar system which employs a plurality of pulsed laser transmitters, a plurality of gated and intensified array camera receivers, an optical scanner for increased field of regard, and a computer for system control, automatic target detection and display generation. U.S. Ser. No. 565,631 (U.S. Pat. No. 5,231,401) provides a means for rapidly searching a large volume of the backscattering medium (e.g., water) for specified targets and improves upon prior art devices in performance as a result of having more energy in each laser pulse (due to simultaneous operation of multiple lasers) and a more sensitive detection system using multiple cameras. The several cameras may be utilized to image different range gates on a single laser pulse or several cameras can be gated on at the same time to provide independent pictures which can then be averaged to reduce the noise level and improve sensitivity. Both of these improvements result in higher signal-to-noise ratio and thus higher probability of detection or greater range of depth capability.
Imaging lidar system of the type hereinabove discussed are often used in conjunction with computerized automatic target detection (ATD) systems for detecting, locating and identifying targets from a plurality of two dimensional images. Examples of such ATD systems are described in commonly assigned U.S. application Ser. Nos. 565,425 (U.S. Pat. No. 5,233,541) and 565,424 (U.S. Pat. No. 5,267,329), both of which were filed on Aug. 10, 1990 and are incorporated herein by reference.
While the imaging lidar systems described above are well suited for their intended purposes (particularly deep ocean detection), there continues to be a need for imaging lidar systems of this type which have improved operational performance and efficiency in the imaging of underwater targets found in shallow water and/or coastal regions. Such coastal zone surveillance presents an even more challenging problem than deep ocean detection. These difficult problems are encountered as a result of the more challenging conditions in shallow water than those encountered previously in deeper water. These factors include highly variable conditions at the air/water interface such as breaking waves, foam and spray, spatially varying bottom depths both parallel and perpendicular to the coastline and the need for high spatial resolution in order to detect and classify small objects which may be partly hidden in the sand.
Notwithstanding the foregoing, the need for imaging lidar systems, suitable for such coastal and shallow water applications is of great interest to government and commercial activities. For example, there continues to be an increasing need for identification of natural and man made obstacles and hazards in the coastal regions. Obvious applications include identifying favorable sites for construction and utilization of shallow water ports and moorings, surveillance of coastal areas to monitor changes in the surf zone as a result of sediment loading or storm activity, and localization and identification of underwater debris as part of search and rescue operations. Other applications involve the detection of obstacles, mines and hidden explosive charges in shallow waters, offshore and in certain riverine environments. Natural and man made underwater effluent and biological fouling of underwater intake and discharge lines such as that caused by the Zebra mussel in the Great Lakes also needs to be identified.